This invention relates to apparatus for the simulation of the acoustic signature produced by a moving vessel received at several receiver locations, each at a different range and bearing to the vessel. Each of the simulated signals must provide a time delayed replica of the vessel's signature corresponding to its range from the receiver, its doppler shifted signature, and its amplitude which reflects the propagation path loss. The time delay is necessary because a maneuver by the vessel which changes its acoustic signal will arrive at the different receivers delayed by the propagation times. The time difference between received signals at the receivers can be used to localize the source. Separate doppler shifts are required because the vessel's velocity will be projected at different angles along the propagation paths to the receivers. The differential doppler shifts can also be used to localize the vessel location relative to the receivers. Finally, the vessel's signature will experience different propagation conditions along the paths to the receivers. Relative signal levels can be used to localize the signal source.
A direct approach to providing simulated signals, which has been used in the past, is to synthesize a separate signature for each receiver. Maneuvers appearing in the signature can be synthesized with the correct time delay. Also, separate doppler shifts and propagation path losses can be applied. Difficulty arises when time delays between random variations within the signature are implemented. Accurate control of these time delays is very difficult. The prior art implementation also requires a synthesizer for each receiver signal being synthesized (there may be as many as eight receiver signals being synthesized simultaneously) at great hardware expense and marginal simulation fidelity.
In contrast, it is an object of the approach employed in this invention to produce the synthesized receiver signals by modeling the physical phenomena as depicted in FIG. 1. An acoustic source 1 generates the signal waveform f(t) produced by the vessel. This waveform is stored in a delay medium for propagation delay storage 2. Access for each receiver is provided to variable delay points within the delay storage 2. By "constantly" varying the delay as a function of range from source to receiver, not only is a delayed replica of the source signal generated, but also a doppler shift or time compression/expansion is produced. The strength of the simulated signal of each receiver is determined by the path loss attenuator 3 in each receiver channel. The process of this invention is exactly the physics of the propagation phenomenon and is not the same as reading out a signal at a faster or slower rate than it was written in. The prior art differential rate technique works well with time limited signals. However, a queuing problem develops when it is applied to continuous signals. This invention does not have a queuing problem since the simulator reads out delayed, doppler-shifted signals at the same rate that they are written in.
In a practical implementation of the invention, there will be a finite resolution to the constantly varying time delay. There are two types of quantization to be considered. First, the value of available time delays will be quantized. Second, the rate at which the time delays are varied will also be quantized. These two quantizations are interrelated in an analogous way as amplitude quantization, and sampling times are related in a normal analog-to-digital converter. The effect of the time delay parameters on the quality of the signal synthesized in one channel is considered.
Spectral analysis has determined that a sinusoidal input signal f(t) with a constant range rate will provide a usable synthesized signal at the doppler frequency if the time delay is updated at least twice during each period of the doppler frequency shift.